852 MILLER [chap. 32 



evolution of photosynthesis other sources of free energy must have been used. 

 It is of interest to consider the sources of such free energy as well as the origin 

 of the appropriate chemical compounds prior to the existence of what should 

 be called living organisms, and before the evolution of photosynthesis. 



Table I 

 Present Sources of Energy Averaged over the Earth 



Source cal cm- 2 yr-i 



Total radiation from sun 260,000 

 Ultraviolet light 



A < 2500 A 570.0 



A < 2000 A 85.0 



A<1500A 3.5" 



Electric discharges 4.0'' 



Cosmic rays 0.0015 



Radioactivity (to 1.0 km depth) O.S*' 



Volcanoes 0.1 3** 



" Inchides the 1.9 cal cm-^ yr-l from the Lyman a at 1216 A 

 (Reuse, 1953). 



^ Includes 0.9 cal cm~2 yr~l from lightning and about 3 cal cm~2 

 yr~l due to corona discharges from pointed objects (Schonland, 

 1953). 



'' The value 4 x 10^ years ago was 2.8 cal cm-2 yr-l (Bullard, 1954). 



'' Calculated assuming the emission of 1 km^ of lava (Cp = 0.25 

 cal/g, p = 3.0 g/cm3) per year at lOOOX'. 



Table I gives a summary of the sources of energy in the terrestrial surface 

 regions. It is evident that sunlight is the principal source of energy, but only a 

 small fraction of this is of wavelengths below 2000 A which can be absorbed 

 by CH4, H2O, NH3, CO2, etc. If more complex molecules were formed, the 

 absorption could move to the 2500 A region or longer wavelengths where sub- 

 stantial energy is available. With the appearance of porphyrins and other 

 pigments, absorption in the visible becomes possible. 



Although probable, it is not certain that the large amount of energy from 

 ultraviolet light would make the principal contribution to the synthesis of 

 organic compounds. Most of the photochemical reactions at these low wave- 

 lengths would take place in the upper atmosphere. The compounds so formed 

 would absorb at longer wavelengths and, therefore, might not get into the 

 oceans before they were decomposed by the ultraviolet light. The question is 

 whether the rate of decomposition in the atmosphere is greater or less than the 

 rate of transport to the oceans. 



Next in importance as a source of energy are electric discharges which, as 

 lightning and corona discharges from pointed objects, occur closer to the Earth's 

 surface; hence more efficient transfer to the oceans would have occurred. 



